The present invention relates generally to equipment utilized in conjunction with a subterranean well and, in an embodiment described herein, more particularly provides a low profile static wellhead plug.
Wellhead plugs which utilize metal to metal seals are well known in the art. It is also well known that relative motion between sealed surfaces in a metal to metal seal is undesirable, for example, because such motion may cause fretting of the metal surfaces, thereby causing the seal to leak. Therefore, a wellhead plug utilizing a metal to metal seal is preferably xe2x80x9cstaticxe2x80x9d, meaning that there is no displacement of its seal in response to pressure applied to the wellhead.
Prior static wellhead plugs are rated for relatively high pressures applied from below, but are rated for relatively low pressures applied from above. A typical wellhead plug uses relatively large lugs engaged with a large profile formed internally in the wellhead to resist high pressure from below. However, the typical static wellhead plug is supported on a small no-go shoulder formed internally in the wellhead. The small shoulder is capable of resisting only relatively low pressures applied to the plug from above. Higher pressures would cause the shoulder material to yield, damaging the plug and/or wellhead, enabling the plug to displace and possibly causing the seal to leak.
To prevent this problem, the no-go shoulder could be increased in size so that higher pressures could be applied to the plug from above, but that would require a smaller drift diameter through the wellhead, or would require a larger overall wellhead, and a larger riser in subsea applications. Neither of these options is desirable, since the former would reduce the bore through the wellhead, and the letter would increase the cost of the wellhead, the riser and their installation.
Therefore, it will readily be appreciated that a need exists for a static wellhead plug which is capable of resisting high pressures from above, as well as from below, which can successfully utilize a metal to metal seal, but which does not require a reduction of a wellhead drift diameter or an enlargement of the wellhead or riser.
In carrying out the principles of the present invention, in accordance with an embodiment thereof, a wellhead plug system is provided which addresses the above problems in the art. The system has a low profile which does not require an enlargement of the wellhead or a reduction of its internal bore, while allowing relatively high pressures to be resisted from above or below.
In one aspect of the present invention, a wellhead plug system is provided which includes a specially adapted wellhead and a corresponding specially constructed wellhead plug. The wellhead has first and second oppositely facing shoulders internally formed on a bore extending through the wellhead. The plug is sealingly received in the bore. A metal to metal seal may be used to seal between the plug and bore.
The plug includes at least one outwardly extendable lug engaging the first shoulder and preventing displacement of the plug relative to the bore in one direction, and at least one outwardly extendable lug engaging the second shoulder and preventing displacement of the plug relative to the bore in an opposite direction. To keep the plug motionless in the bore, the first lug engages the first shoulder while the second lug is engaged with the second shoulder.
Preferably, the lugs are biased into contact with the shoulders so that compression or tension is induced in the plug between the lugs. This is accomplished in one embodiment by maintaining one lug in contact with a shoulder while another lug is biased into contact with another shoulder. The shoulders are laterally inclined, so this biasing contact wedges the lugs between the shoulders, thereby compressing a portion of the plug between the lugs. Other embodiments could induce tension in the plug between the lugs.
Instead of separate lugs for contacting opposing shoulders in the wellhead, the plug could use one or more lugs, each of which contacts both of the opposing shoulders. In this manner, each lug would act to prevent movement of the plug in both directions relative to the wellhead. Lugs utilized with the invention may have a variety of shapes, including polygonal, circular, etc.